An organic EL display panel is a display panel having a light-emitting device using electroluminescence of an organic compound. An organic EL display panel has an EL device including a cathode, an anode, and an electroluminescent organic compound layer disposed between both electrodes. The electroluminescent organic compound can be approximately classified into a combination of low-molecular organic compounds (a host material and a dopant material) and a high-molecular organic compound.
Examples of the electroluminescent high-molecular organic compound include poly(p-phenylene vinylene) referred to as PPV or derivatives thereof. An organic EL display panel using the electroluminescent high-molecular organic compound is characterized in that it can be driven with a relatively-low voltage and it has low power consumption.
An electroluminescent organic compound can be dissolved in an organic solvent to form an ink. For example, a high-molecular organic compound can be dissolved in an aromatic organic solvent such as xylene or toluene to produce an ink. By producing an ink containing an electroluminescent organic compound, an organic light-emitting layer can be formed through a printing technique such as an inkjet method. As a result, it is being considered for easily addressing an increase in size of a screen of a display panel and research and studies thereof have been actively carried out.
An electroluminescent high-molecular organic compound is disposed in each pixel through a printing technique such as an inkjet method depending on colors (red, green, and blue) of emitted light. For example, a polymer ink including a high-molecular organic compound and a solvent is ejected from an inkjet head and is printed on each pixel. When a polymer ink is printed in each pixel, the polymer ink should not permeate into neighboring pixels.
The following two methods are employed so as not to cause permeation of the polymer ink into neighboring pixels.
In a first method, a partition wall (bank) defining pixels is provided and the polymer ink is accurately printed on the respective pixels. Accordingly, it is possible to suppress the permeation of the ink into neighboring pixels (for example, see Patent Document 1).
In a second method, pixels are arranged in line regions defined by line banks and the polymer ink is printed in line regions. At this time, the thickness of an organic light-emitting layer may become smaller at edges of pixel electrodes arranged in the line regions defined by line banks, and thus short-circuit between the pixel electrode and a counter electrode which are disposed on the organic light-emitting layer may occur. In this regard, it is known that the short-circuiting between the pixel electrode and the counter electrode can be prevented by disposing an insulating layer of silicon oxide or the like to cover the edges of the pixel electrodes arranged in the line regions defined by the line banks (for example, see Patent Documents 2 and 3).
An organic EL device (for example, see Patent Document 4) in which edges of pixel electrodes are directly covered with an insulating layer on the pixel electrodes, an organic EL device (for example, see Patent Documents 5 to 8) in which edges of pixel electrodes are covered with an insulating inorganic layer such as a bank directly or with a hole injection layer interposed therebetween, and the like have been proposed. In addition, a color filter including a bank partitioning a colorant layer on a transparent substrate and a black matrix covering the bank and edges of the colorant layer is known (for example, see Patent Document 9).
FIG. 5 is an oblique view of an organic EL device 10 described in Patent Document 2. Insulating layer 220 is formed in both a direction parallel to line bank 230 and a direction perpendicular thereto so as to cover the edges of pixel electrodes 210. In FIG. 5, reference numeral 100 represents a substrate, reference numeral 240 represents a second bank, and reference numeral 300 represents a pixel area.
The polymer ink can be more easily and rapidly printed on the pixels through the second method of printing the polymer ink in the line regions where the pixels are arranged, rather than the first method of printing the polymer ink in the respective regions where one pixel is arranged. The uniformity in thickness of the organic light-emitting layer formed on the pixels formed through the second method is superior to that through the first method.
This is because the polymer ink at edges of the banks is attracted by surface tension. Accordingly, the uniformity in thickness of the organic light-emitting layer can deteriorate more easily. Therefore, an organic light-emitting layer having higher uniformity in thickness can be generally formed through the second method in which only two sides of a pixel are surrounded with the banks, rather than the first method in which four sides of a pixel are surrounded with the banks.
When the organic light-emitting layer is formed through an inkjet method, a difference in ejection volume among nozzles of an inkjet head may occur. The difference in ejection volume among the nozzles directly causes a difference in thickness of the organic light-emitting layer. When the ink is ejected in the pixel region or the line region, the ink through more nozzles is ejected in the line region and the ink through less nozzles is ejected in the pixel region. Accordingly, by ejecting ink in the line region, it is possible to reduce the influence on the difference in ejection volume among the nozzles. From this point of view, the second method has recently been studied in some degree.